Introduction of El Niño

El Niño events are introduced by weakened trade winds over the equator. Trade winds blow west restricting the amount of heat transported to the east. During El Niño events, the winds blowing west are much smaller thus the ocean currents to the east are stronger (Central America), which in turn brings warm and wet weather to South America.

On the west side of Pacific, however, things are more complicated. El Niño impacts vary among different geographic regions and El Niño types. A single El Niño event may bring drought to one country while increasing rainfall in another. This is largely dependent on the way that El Niño moves the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). For these Pacific Island countries, lying inside or outside of the convergence zones determines whether they experience drought or wet weathers. (ITCZ and SPCZ are both atmospheric patterns that affect temperature and rainfall over Equatorial Pacific countries. ITCZ mainly control countries north to the equator while SPCZ is responsible for countries in the Southwest Pacific.)

Sea Surface Temperature Patterns

Scientists are interested in seeing how various types of El Niño events affect Pacific Island countries. They set up stations in these countries and compared the data from these stations to other large scale datasets.

Figure 1. Countries studied.

In order to monitor El Niño events, sea surface temperature anomalies are being recorded in the following three regions: cold tongue El Niño (CTE, Niño-3, 150˚–90˚W), mixed El Niño(ME, Niño-3.4, 170˚–120˚W), and warm pool El Niño (WPE, Niño-4 ,160˚E–150˚W), all over 5˚S–5˚N. An El Niño year is considered to be one in which the Sep-Feb mean of any of these indices exceeds the 1950-2011 standard deviation. The maximum anomalies are in eastern Pacific for CTE, the central Pacific for ME, and western Pacific for WPE. As shown in figure 3, CTE and La Niña have the strongest and most robust SST anomalies. (La Niña can be considered as a negative phase of El Niño meaning it has about the opposite temperature and precipitation effect from El Niño.)

Wind and Precipitation Patterns

La Niña events show strengthened trade winds while El Niño events show weakening westerly wind anomalies to varying degrees. Again CTE appears to have stronger wind anomalies than the other two types. This eastward wind anomaly agrees with eastward positive sea surface temperature too. CTE events also show strong anomalous convergence of surface winds in the western and central Pacific coinciding with the positions of the ITCZ and SPCZ.

The response of rainfall shows a very clear relationship to the sea surface temperature anomalies and wind changes. During CTE, large and extensive rainfall changes occur. There are increased rainfall to the east of Solomon Islands and decreased rainfall to the west. Patterns with ME and WPE are similar but the areas that exert significant anomalies from historical mean are smaller for WPE. Meanwhile La Niña events are basically the opposite of ME events. Overall countries that experiences suppressed rainfall in ME also receive enhanced rainfall in La Niña.

Figure 4. Rainfall anomalies.

These distinct rainfall anomalies between different El Niño types indicate that countries near the equatorial Pacific experience different impacts from these El Niño as well. Specific countries are being taken into consideration when studying this phenomenon. For example, Nauru and Tarawa have a clear trend of enhanced or suppressed rainfall by El Niño and La Niña respectively. Majuro (Marshall Islands) reversely tend to have dry El Niño years and wet La Niña years. (Countries locations are indicated in figure 1.)

Country-Specific Studies of El Niño

SST anomalies generally show cooling during El Niño and warming for La Niña events in the countries in the far west, southwest, and northwest Pacific. Opposite changes are seen closer to the equator east of about 170 ̊E. For rainfall, different responses among the El Niño types are even more dramatic. For instance, the countries from the Solomon Islands to Samoa have quite strong rainfall reductions in November–April in CTE years while they only experience weak anomalies during other El Niño years.

It was found that CTE events are associated with the greatest rainfall impacts in most countries over Pacific. In northwest and southwest Pacific countries, CTE events generally have the same sign of rainfall changes as the other events. However, for some countries in equatorial Pacific, CTE brings about the opposite rainfall changes as the other El Niño events.

Since El Niño impacts on the west Pacific normally occur several months earlier than the east, it helps to predict El Niño by detecting signals in the west at an early stage. Also, the Pacific Islands are among those countries suffering the most from El Niño. It is important to understand the possible consequences of different types of El Niño in order to help reduce the damage.

Caoxin is a graduate student in the Graduate School of Oceanography at the University of Rhode Island. Her research interest lies in persistent organic pollutants in the environment. When she is not doing research she likes to create new cuisines.